A horse can destroy a human in a sprint, but over a brutal long haul in summer heat, Daniel Lieberman argues the advantage can flip: humans keep cooling themselves through sweat, while a horse’s larger body struggles to shed heat fast enough.

In a sprint, it is no contest. A horse would leave a human far behind before either had gone a hundred metres. But over a long, punishing distance in summer heat, the Harvard physiologist Daniel Lieberman argues, the advantage can flip. Humans keep cooling themselves by sweating as they move, while a horse’s much larger body struggles to shed heat fast enough, and the animal built for speed can be run into the ground by the one built for stamina.

It is a striking idea, and one with real evidence behind it, though, as we will see, it is also a hypothesis that not everyone accepts.

The sprint is no contest

Start with the obvious. Horses can gallop at something like 55 to 70 kilometres an hour, while even the fastest human sprinter tops out near 37, and only for a few seconds. Over any short race the horse wins easily, and by a distance that is almost embarrassing. As sprinters, humans are unremarkable animals.

What we are unusually good at is something else entirely: covering long distances at a steady pace, and doing it in the heat.

Where humans are exceptional

This is the heart of what is often called the endurance running hypothesis, an idea first raised in the 1980s and later developed and popularised by Lieberman and colleagues. It holds that natural selection shaped the human body for long-distance running. The evidence is written into our anatomy: long legs, short toes, arched feet that act as springs, a long Achilles tendon that stores and returns energy, large muscles in the buttocks that stabilise the trunk, and mechanisms that keep the head steady as we run.

Put together, these traits make humans mediocre sprinters but extraordinary distance movers, capable of jogging for hours over terrain that would exhaust most other animals.

The cooling advantage

The decisive piece, though, is how we handle heat. Humans are nearly hairless and carry millions of sweat glands across the whole surface of the body, so we cool ourselves by evaporating sweat continuously, over our entire skin, even while moving at speed.

Running upright helps too. Because we move on two legs, our breathing is not tied to the rhythm of our stride, so we can breathe freely and keep dumping heat whatever pace we hold. Most four-legged runners cannot. That combination, whole-body sweating plus decoupled breathing, lets a human keep going in conditions that force other animals to stop and rest.

What about the horse

It is worth being fair to the horse, because the popular version of this story is often too crude. Horses are not helpless in the heat. They are, in fact, among the minority of mammals that sweat heavily, so it is not a simple case of us sweating and them not.

Two things still work against a galloping horse over a long, hot haul. The first is size. A large body generates more heat and has relatively less surface area to shed it from, so it warms up faster and cools more slowly than a smaller one. The second is gait. When a horse gallops, its breathing is locked to the pounding rhythm of its stride, which limits how freely it can pant to cool down while running hard. Pushed for long enough in the heat, a horse has to slow to avoid overheating. A human can simply keep jogging.

Does it actually happen?

This is not only theory. In Wales, an annual Man versus Horse race pits runners against riders over roughly 22 miles of rough country. The horse usually wins, but on hot, hard years human runners have beaten the entire field outright, exactly as the physiology predicts.

Lieberman’s wider case rests on persistence hunting, in which hunters chase an animal through the midday heat, never fast enough to catch it in a burst, but relentlessly enough that it cannot stop to cool down, until it collapses from overheating and can be walked up to. Such hunts have been documented among peoples such as the San of the Kalahari, pursuing prey over 20 to 40 kilometres until the animal succumbs. The argument is that this ability to run prey down in the heat may have been an important part of how our ancestors fed themselves.

A hypothesis, not a settled fact

Here honesty is due. The endurance running hypothesis is influential but not universally accepted. Persistence hunting is real, but it is rare in the ethnographic record, and critics argue it may have been too uncommon to have driven the evolution of the human body as strongly as the theory suggests. The anatomy and the cooling advantages are not in dispute. What remains debated is how central running down prey really was to becoming human.

Why it matters

Either way, the idea reframes what makes the human body remarkable. It is not strength, and it is certainly not speed. It is stamina and the ability to stay cool, packaged in a sweaty, upright, long-legged frame that is built for the marathon rather than the dash.

So the horse wins the sprint every single time. But send both out across an open plain on a scorching afternoon, and the slow, sweating, tireless human is, remarkably, the one with a chance of still moving when the sun goes down.